Linear accelerator

ABSTRACT

This device allows the variation of the coupling between two points in an RF circuit in a very simple way while maintaining the RF phase relationship and varying the relative magnitude of the RF fields. The device is characterized by a simple mechanical control of coupling value, that has negligible effect on the phase shift across the device. This is achieved by the simple rotation of the polarisation of a TE 111  mode inside a cylindrical cavity. Such a device does not contain resistive elements, and the sliding mechanical surfaces are free from high RF currents. This device finds an application in standing wave linear accelerators, where it is desirable to vary the relative RF field in one set of cavities with respect to another, in order that the accelerator can operate successfully over a wide range of energies.

BACKGROUND FIELD OF THE INVENTION

The present invention relates to a linear accelerator.

BACKGROUND ART

Linear accelerators, particularly of the standing wave design, are knownas a source of an electron beam, for example for use in X-Raygeneration. This beam can be directed to an X-ray target which thenproduces suitable radiation. A common use for such X-rays or for theelectron beam is in the medical treatment of cancers etc.

It is often necessary to vary the incident energy of the electron beamon the X-ray target. This is particularly the case in medicalapplications where a particular energy may be called for by thetreatment profile. Linear standing wave accelerators comprise a seriesof accelerating cavities which are coupled by way of coupling cavitieswhich communicate with an adjacent pair of accelerating cavities.According to U.S. Pat. No. 4,382,208, the energy of the electron beam isvaried by adjusting the extent of rf coupling between adjacentaccelerating cavities. This is normally achieved by varying thegeometrical shape of the coupling cavity.

This variation of the geometrical shape is typically by use of slidingelements which can be inserted into the coupling cavity in one or morepositions, thereby changing the internal shape of the cavity. There area number of serious difficulties with this approach arising from thevarious other resonant parameters that are dictated by the cavitydimensions. Often more than one such element has to be moved in order topreserve the phase shift between cavities at a precisely defined value.The movement of the elements is not usually identical, so they have tobe moved independently, yet be positioned relative to each other and thecavity to very great accuracy in order that the desired phaserelationship is maintained. Accuracies of ±0.2 mm are usually required.This demands a complex and high-precision positioning system which isdifficult to engineer in practice. In those schemes which have less thantwo moving parts (such as that proposed in U.S. Pat. No. 4,286,192), thedevice fails to maintain a constant phase between input and output,making such a device unable to vary RF fields continuously, and are thusreduced to the functionality of a simple switch. They are in fact oftenreferred to as an energy switch.

Many of these schemes also propose sliding contacts which must carrylarge amplitude RF currents. Such contacts are prone to failure by weldinduced seizure, and the sliding surfaces are detrimental to the qualityof an ultra high vacuum system. Issues of this nature are key to makinga device which can operate reliably over a long lifetime.

The nature of previous proposed solutions can be summarised as cavitycoupling devices with one input and one output hole, the whole assemblyacting electrically like a transformer. To achieve variable couplingvalues the shape of the cavity has had to be changed in some way, bymeans of devices such as bellows, chokes and plungers. However the priorart does not offer any device which can vary the magnitude of thecoupling continuously over a wide range by means of a single axiscontrol, while simultaneously maintaining the phase at a constant value.

The present state of the art is therefore that such designs are acceptedas providing a useful way of switching between two predeterminedenergies. However, it is very difficult to obtain a reliable acceleratorusing such designs that offers a truly variable energy output.

A good summary of the prior art can be found in U.S. Pat. No. 4,746,839.

SUMMARY OF THE INVENTION

The present invention therefore provides a standing wave linearaccelerator, comprising a plurality of resonant cavities located along aparticle beam axis, at least one pair of resonant cavities beingelectromagnetically coupled via a coupling cavity, the coupling cavitybeing substantially rotationally symmetric about its axis, but includinga non-rotationally symmetric element adapted to break that symmetry, theelement being rotatable within the coupling cavity, that rotation beingsubstantially parallel to the axis of symmetry of the coupling cavity.

In such an apparatus, a resonance can be set up in the coupling cavitywhich is of a transverse nature to that within the acceleratingcavities. It is normal to employ a TM mode of resonance with theaccelerating cavities, meaning that a TE mode, such as TE₁₁₁, can be setup in the coupling cavity. Because the cavity is substantiallyrotationally symmetric, the orientation of that field is not determinedby the cavity. It is instead fixed by the rotational element.Communication between the coupling cavity and the two acceleratingcavities can then be at two points within the surface of the couplingcavity, which will “see” a different magnetic field depending on theorientation of the TE standing wave. Thus, the extent of coupling isvaried by the simple expedient of rotating the rotational element.

Rotating an element within a vacuum cavity is a well known art and manymethods exist to do so. This will not therefore present a seriousengineering difficulty. Furthermore, eddy currents will be confined tothe rotational element itself and will not generally need to bridge theelement and its surrounding structure. Welds will not therefore presenta difficulty.

The design is also resilient to engineering tolerances. Preliminarytests show that an accuracy of only 2 dB is needed in order to obtain aphase stability of 2% over a 40° coupling range. Such a rotationalaccuracy is not difficult to obtain.

It is preferred if the rotational element is freely rotatable within acoupling cavity of unlimited rotational symmetry. This arrangement givesan apparatus which offers greatest flexibility.

A suitable rotational element is a paddle disposed along the axis ofsymmetry. It should preferably be between a half and three quarters ofthe cavity width, and is suitably approximately two-thirds of the cavitywidth. Within these limits, edge interactions between the paddle and thecavity surfaces are minimised.

The axis of the resonant cavity is preferably transverse to the particlebeam axis. This simplifies the rf interaction considerably.

The accelerating cavities preferably communicate via ports set on asurface of the coupling cavity. It is particularly preferred if theports lie on radii separated by between 40° and 140°. A more preferredrange is between 60° and 120°. A particularly preferred range is between80 and 100°, i.e. approximately 90°.

The ports can lie on an end face of the cavity, i.e. one transverse tothe axis of symmetry, or on a cylindrical face thereof. The latter islikely to give a more compact arrangement, and may offer greatercoupling.

Thus, the invention proposes the novel approach of coupling adjacentcells via a special cavity operating in a TE mode, particularly theTE₁₁₁ mode. By choosing the coupling positions of the input and outputholes to lie along a chord of the circle forming one of the end walls ofthe cavity, a special feature of the TE₁₁₁ mode can be exploited torealise a coupling device with unique advantages. Instead of changingthe shape of the cavity, this invention proposes to rotate thepolarisation of TE₁₁₁ mode inside the cavity by means of a simplepaddle. Because the frequency of the TE₁₁₁ mode does not depend upon theangle that the field pattern makes with respect to the cavity (thepolarising angle), the relative phase of RF coupled into two points isinvariant with respect to this rotation, at least over 180°. At the sametime, the relative magnitude of the RF magnetic fields at the twocoupling holes lying along a chord varies by up to two orders ofmagnitude. This property of the RF magnetic field is the basis of thevariable RF coupler of this invention.

The key to the proposed device is that the moving paddle is not a deviceto change the shape of the cavity, as described in the prior art, but ismerely a device to break circular symmetry of the cylindrical cavity. Assuch the paddle does not have to make contact with the walls of thecavity, nor does any net RF current flow between the paddle and thecavity wall. This makes the device simple to construct in vacuum,requiring only a rotating feed-through, which is well known technology.Alternatively, the paddle might be rotated by an external magneticfield, and so eliminate the vacuum feed-through requirements entirely.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a view of the electric field lines of the TE₁₁₁ cylindricalcavity mode;

FIG. 2 shows a longitudinal cross-section through a standing wave linearaccelerator according to a first embodiment of the present invention;

FIG. 3 shows a section on III—III of FIG. 2;

FIG. 4 is a longitudinal cross-section through a standing wave linearaccelerator according to a second embodiment of the present invention;

FIG. 5 is a section on V—V of FIG. 4;

FIG. 6 is a perspective view of an accelerator element of a thirdembodiment of the present invention;

FIG. 7 is an axial view of the embodiment of FIG. 6;

FIG. 8 is an exploded view of the embodiment of FIG. 6;

FIG. 9 is a section on IX—IX of FIG. 7;

FIG. 10 is a section on X—X of FIG. 7;

FIG. 11 is a perspective view of a fourth embodiment of the presentinvention;

FIG. 12 is a view of the embodiment of FIG. 11 along the acceleratoraxis;

FIG. 13 is a section on XIII—XIII of FIG. 12; and

FIG. 14 is a section of XIV—XIV of FIG. 12.

DETAILED DESCRIPTION OF THE EXAMPLES

In a standing wave accelerator the device could be implemented as shownin the first embodiment, FIGS. 2 and 3. These show three on-axisaccelerating cells 10, 12, 14 as part of a longer chain of cavities. Thefirst and second accelerating cavities 10, 12 are coupled together witha fixed geometry coupling cell 16, which is known art. Between thesecond and third on-axis cavities 12, 14, the fixed geometry cell isreplaced by a cell 18 according to the present invention. This cell 18is formed by the intersection of a cylinder with the tops of the archesthat make up the accelerating cells thus forming two odd shaped couplingholes 26, 28. To function as intended, these holes should ideally bealong a (non-diametrical) chord of the off-axis cylinder, which impliesthat the center line of the cylinder is offset from the center line ofthe accelerator, as shown in the FIG. 3. These coupling holes are inregion of the cavity where magnetic field dominates, and so the couplingbetween cells is magnetic. However unlike the fixed geometry cells thereis now a simple means of varying the coupling between cells, andconsequently the ratio of the RF electric field in the second and thirdon-axis cells. The strength of the coupling (k) depends upon the shapeof the hole and the local value of the RF magnetic field at the positionof the hole. The on-axis electric field varies inversely with the ratioof the k values. Hence:

E ₁ /E ₂ =k ₂ /k ₁

The magnetic field pattern close to the end wall means that if thecoupling holes lie along a chord, k₁ will increase as k₂ decreases.

A rotatable paddle 20 is held within the cavity 18 by an axle 22 whichin turn extends outside the cylindrical cavity 18. As shown in FIG. 2,the axle has a handle 24 to permit rotation of the paddle 20, but thehandle could obviously be replaced by a suitable actuator.

The paddle serves to break the symmetry of the cavity 18, thus forcingthe electric lines of field to lie perpendicular to the paddle surface.

The end result is a device which has just one simple moving part, whichupon rotation will provide a direct control of the coupling betweencells, while at the same time keeping the relative phase shift betweeninput and output fixed, say at a nominal π radians. The only degree offreedom in the system is the angle of rotation of the paddle. In atypical standing wave accelerator application this would only have to bepositioned to the accuracy of a few degrees. Such a control would allowthe energy of a linear accelerator to be adjusted continuously over awide range of energy.

According to the second embodiment, shown in FIGS. 4 and 5, the couplingcavity 30 is still transverse to the longitudinal axis of theaccelerating cavities, but intersects with accelerating cavities 12, 14along a cylindrical face thereof. Thus, the axes of the accelerator andof the coupling cavity do not intersect, but extend in directions whichare mutually transverse. The paddle 20 etc. is unchanged. Otherwise, theoperation of this embodiment is the same as the first.

FIGS. 6-10 illustrate a third embodiment of the present invention. Inthe Figures, a short sub-element of a linear accelerator is illustrated,consisting of two accelerating cavities and the halves of two couplingcavities either side. In addition, the element includes a singlecoupling cavity embodying the present invention, joining the twoaccelerating cavities. A complete accelerator would be made up ofseveral such sub-elements joined axially.

In FIG. 6, the axis 100 of the accelerating cavities passes into a smallopening 102 into a first coupling cavity 104 (not visible in FIG. 6). Afurther accelerating cavity 108 communicates with the first acceleratingcavity 104 via an aperture 106. The second cavity 108 then has a furtheraperture 110 on its opposing side to communicate with subsequentaccelerating cavities formed when the sub-element of this embodiment isrepeated along the axis 100. Thus, a beam being accelerated passes inorder through apertures 102, 106, 110 etc.

A pair of coupling half-cavities are formed in the illustratedsub-element. The first half cavity 112 provides a fixed magnitudecoupling between the first accelerating cavity 104 and an adjacentaccelerating cavity formed by an adjacent sub-element. This adjacentsub-element will provide the remaining half of the coupling cavity 112.Likewise, the second coupling cavity 114 couples the second accelerating108 to an adjacent cavity provided by an adjacent element. Each couplingcavity includes an upstanding post 116, 118 which tunes that cavity toprovide the appropriate level of coupling desired. The coupling cavities112, 114 are conventional in their construction.

The first accelerating cavity 104 is coupled to the second acceleratingcavity 108 via an adjustable coupling cavity 120. This consists of acylindrical space within the element, the axis of the cylinder beingtransverse to the accelerator axis 100 and spaced therefrom. The spacingbetween the two axes at their closest point and the radius of thecylinder is adjusted so that the cylinder intersects the acceleratingcavities 104, 108, resulting in apertures 122, 124. As illustrated inthis embodiment, the cylinder 120 is positioned slightly closer to thesecond accelerating cavity 108, making the aperture 124 larger than theaperture 122. Depending on the design of the remainder of theaccelerator, this may in certain circumstances be beneficial. However,it is not essential and in other designs may be less desirable.

At one end of the adjustable coupling cavity 120, an aperture 126 isformed to allow a shaft 128 to pass into the interior of the cavity. Theshaft 128 is rotatably sealed in the aperture 126 according to knownmethods. Within the adjustable cavity 120, the shaft 128 supports apaddle 130 which is therefore rotationally positionable so as to definethe orientation of a TE₁₁₁ field within the adjustable coupling cavity120 and thus dictate the amount of coupling between the first cavity 104and the second cavity 108.

Cooling channels are formed within the element to allow water to beconducted through the entire construction. In this example, a total offour cooling channels are provided, equally spaced about theaccelerating cavities. Two cooling channels 132, 134 run above and belowthe fixed coupling cavities 112, 114 and pass straight through the unit.Two further coupling cavities 136, 138 run along the same side as thevariable cavity 120. To prevent the cooling channels conflicting withthe accelerating cavities 104, 108 or the adjustable coupling cavity120, a pair of dog legs 140 are formed, as most clearly seen in FIGS. 7and 8.

FIG. 8 shows an exploded view of the example illustrating the manner inwhich it can constructed. A central base unit 150 contains the couplingcavity and two halves of the first and second accelerating cavities 104,108. The two accelerating cavities can be formed by a suitable turningoperation on a copper substrate, following which the centralcommunication aperture 106 between the two cavities can be drilled out,along with the coolant channels 132, 134, 136, 138 and the dog leg 140of the channels 136 and 138. The adjustable coupling cavity 120 can thenbe drilled out, thereby forming the apertures 122 and 124 between thatcavity and the two accelerating cavities 104, 108. Caps 152, 154 canthen be brazed onto top and bottom ends of the adjustable couplingcavity 120, sealing it.

End pieces 156, 158 can then be formed for attachment either side of thecentral unit 150 by a brazing step. Again, the remaining halves of thecoupling cavities 104, 108 can be turned within these units, as can thehalf cavities 112, 114. Coolant channels 132, 134, 136 and 138 can bedrilled, as can the axial communication apertures 102, 110. The endpieces can then be brazed in place either side of the central unit,sealing the accelerating cavities and forming a single unit.

A plurality of like units can then be brazed end to end to form anaccelerating chain of cavities. Adjacent pairs of accelerating cavitieswill be coupled via fixed coupling cavities, and each member of suchpairs will be coupled to a member of the adjacent pair via an adjustablecoupling cavity 120.

The brazing of such units is well known and simply involves clampingeach part together with a foil of suitable eutectic brazing alloytherebetween, and heating the assembly to a suitable elevatedtemperature. After cooling, the adjacent cavities are firmly joined.

FIGS. 11-14 illustrate a fourth example of the present invention. Aswith the third example, this example illustrates a sub-element of alinear accelerator containing two accelerating cavities. A plurality ofsub-element as illustrated can be joined end to end to produce a workingaccelerator.

A pair of accelerating cells 204, 208 are aligned along an accelerationaxis 200. An aperture 202 allows an accelerating beam to enter theaccelerating cavity 204 from an adjacent element, while an aperture 206allows the beam to continue into accelerating cavity 208, and anaperture 210 allows the beam to continue on the axis 200 out of theaccelerating cavity 208 into a further cavity.

An adjustable coupling cavity 220 is formed, interconnecting the twocavities 204 and 208. This adjustable coupling cavity 220 consists of acylinder whose axis is transverse to the accelerator axis 200 and spacedtherefrom. The radius of the cylinder and the positioning of the axisare such that it intersects with the accelerating cavities 204, 208,thereby forming communication apertures 222, 224. As illustrated, theadjustable coupling cavity 220 is positioned more closely to theaccelerating cavity 204, and therefore the aperture 222 is slightlylarger than the aperture 224. However, this is not essential in allcircumstances and depends on the construction of the remainder of theaccelerator.

The cylinder forming the adjustable coupling cavity 220 has end faces260, 262 which are linearly adjustable along the axis of the cylinder220. Thus, the length of the coupling cavity can be varied in order tomatch the external design of the accelerator. This length needs to beset according to the resonant frequency of the accelerator. However,experimental work shows that the setting does not need to be especiallyprecise.

The end wall 262 includes an axial aperture 226, through which passes anaxle 228. A handle 264 is formed on the outside of the wall 262, and apaddle 230 is formed on the inner face. That paddle serves to break therotational symmetry of the adjustable coupling cavity 220 and therebyfix the orientation of the TE₁₁₁ field. Thus, the orientation of thefield, and hence the magnitude of coupling, can be varied by adjustingthe handle 264. Clearly a suitable mechanical actuator could be employedinstead of a manually adjustable handle.

It has been found that adjustable coupling cavities such as thosedescribed in the third and fourth embodiments are capable of providing acoupling co-efficient between the two accelerating cavities of between 0and 6%. Most designs of accelerator require a coupling co-efficient ofup to 4%, and therefore this design is capable of providing thenecessary level of coupling for substantially all situations.

Through the present invention, a continuous range of coupling constantscan be obtained without disrupting the phase shift between acceleratingcavities. Furthermore, the third embodiment allows a viable acceleratorto be constructed from easily manufactured elements.

It will of course be appreciated by those skilled in the art that theabove-described embodiment is simply illustrative of the presentinvention, and that many variations could be made thereto.

What is claimed is:
 1. A standing wave linear accelerator, comprising aplurality of resonant cavities located along a particle beam axis, atleast one pair of resonant cavities being electromagnetically coupledvia a coupling cavity, the coupling cavity being substantiallyrotationally symmetric about its axis, but including a non-rotationallysymmetric element adapted to break that symmetry, the element beingrotatable within the coupling cavity, that rotation being substantiallyparallel to the axis of symmetry of the coupling cavity.
 2. Anaccelerator according to claim 1 in which communication between thecoupling cavity and the two accelerating cavities is respectively at twopoints within the surface of the coupling cavity.
 3. An acceleratoraccording to claim 1 wherein the rotational element is freely rotatablewithin a coupling cavity of unlimited rotational symmetry.
 4. Anaccelerator according to claim 1, in which the rotational element is apaddle disposed along the axis of symmetry.
 5. An accelerator accordingto claim 4 wherein the paddle occupies between a half and three quartersof the cavity width.
 6. An accelerator according to claim 1, wherein theaxis of the resonant cavity is transverse to the particle beam axis. 7.An accelerator according to claim 1, wherein the accelerating cavitiescommunicate via ports set on a surface of the coupling cavity.
 8. Anaccelerator according to claim 1, wherein the ports lie on radii of thecoupling cavity separated by between 40° and 140°.
 9. An acceleratoraccording to claim 1, wherein the ports lie on radii of the couplingcavity separated by between 60° and 120°.
 10. An accelerator accordingto claim 1, wherein the ports lie on radii of the coupling cavityseparated by between 80° and 100°.
 11. An accelerator according to claim1, wherein the ports lie on an end face of the cavity.
 12. Anaccelerator according to claim 1, wherein the ports lie on a cylindricalface of the cavity.